The present invention relates to a cryogenic rectification process and apparatus for separating high and low volatility components of a gaseous mixture wherein the mixture is initially compressed and then cooled to a temperature suitable for its rectification. More particularly, the present invention relates to such a process and apparatus in which the low volatility component is pumped to a delivery pressure and then is vaporized within a main heat exchanger used in cooling the mixture. Even more particularly, the present invention relates to such a process and apparatus in which thermodynamic irreversibilities within the main heat exchanger are minimized.
Components of gaseous mixtures having different volatilities are separated from one another by a variety of well-known cryogenic rectification processes. Such processes utilize a main heat exchanger to cool the gaseous mixture to a temperature suitable for rectification alter the gaseous mixture has been compressed. The rectification is carried out in distillation columns incorporating trays or packing (structured or random) to bring liquid and gaseous phases of the mixture into intimate contact and thereby separate the components of the mixture in accordance with their volatilities. In order to avoid the use of a product compressor to produce the lower volatility component at a delivery pressure, the distillation is carried out such that the lower volatility component is produced in liquid form. The lower volatility component in the liquid form is then pumped to the delivery pressure and vaporized within the main heat exchanger.
An important cryogenic rectification process concerns the separation of air. Air contains a lower volatility component, oxygen, and a higher volatility component, nitrogen. In the production of pressurized oxygen gas, a liquid oxygen product of the cryogenic rectification of air is pumped to a delivery pressure and heated by incoming air in a heat exchanger from which it emerges as a pressurized gas. Typically, at least part of the air feed must be pressurized to a much higher pressure than the oxygen in order to provide the appropriate temperature difference in the heat exchange. For instance, when an oxygen product, which amounts to 19-22% of the incoming air by volume percent is pumped to 42.8 bar(a), about 35-40% of the incoming air is compressed to about 74.5 bar(a). This requirement is a result of the non-conformity in the temperature and the heat transferred between the feed air and the product streams in some parts of the main heat exchanger, which affects the warning up of the products and the cooling down of the air. Concurrently, wide temperature differences exist between the air and the product streams in part of the heat exchanger. This is known as thermodynamic irreversibility and increases the energy requirement of the process.
As will be discussed, the present invention provides a process and apparatus for the separation of air in which thermodynamic irreversibilities in the main heat exchanger are minimized. Additionally, the present invention also relates to a method of vaporizing a pumped low volatility product within a main heat exchanger, for instance, components of air, petrochemicals and etc. such that thermodynamic irreversibilities within the main heat exchanger are minimized.